Magnetic domain devices are based on the nucleation and propagation of magnetic domains in a layer of magnetic material such as, e.g., a magnetic garnet material deposited on a nonmagnetic garnet substrate. Desired magnetic anisotropy in such a layer results in an "easy direction" of magnetization which is perpendicular to the layer and which renders the layer capable of sustaining magnetic domains whose magnetization is antiparallel to the magnetization of layer material surrounding the domains.
Evaluation of a material for suitability as a magnetic domain material typically involves the consideration of a number of material parameters such as, e.g., saturation magnetization, uniaxial anisotropy, exchange constant, material length parameter, bubble collapse field, anisotropy field, quality factor, coercivity, temperature dependence of the bubble collapse field, lattice parameter, magnetostriction coefficient, and cubic anisotropy as described, e.g., by S. L. Blank et al., "Design and Development of Single-Layer, Ion-Implantable Small-Bubble Materials for Magnetic Bubble Devices", Journal of Applied Physics, Vol. 50, No. 3 (March 1979), pp. 2155-2160.
In the following, attention is directed primarily to the determination of the magnetic anisotropy field which is commonly designated as H.sub.K (in units of oersteds) and which may be defined as the weakest magnetic field which is capable of switching the magnetization of a layer from the easy direction perpendicular to the layer to a direction in the plane of the layer.
A standard method for experimentally determining the magnetic anisotropy field is based on ferromagnetic resonance techniques as described by R. C. Le Craw et al., "Temperature Dependence of Growth-Induced Magnetic Anistropy in Epitaxial Garnet Films by Resonance Techniques", AIP Conference Proceedings No. 5 (Magnetism and Magnetic Materials--1971), American Institute of Physics, 1972, pp. 200-204. Alternate methods have been proposed which promise to be more expeditious and which are based on the observation of a certain regular pattern of bubbles upon release of an applied magnetic field which then is considered to correspond to the anisotropy field. Such methods have been proposed by a number of authors such as, e.g.,
A. J. Kurtzig et al., "Noncubic Magnetic Anisotropies in Bulk and Thin-Film Garnets", IEEE Transactions on Magnetics MAG-7 (1971), pp. 473-476; PA1 A. B. Smith et al., "Uniaxial Magnetic Anisotropy in ErEu-, YEu-, and YGdTm-Garnet Films for Bubble-Domain Devices", AIP Conference Proceedings No. 10 (Magnetism and Magnetic Materials--1972), American Institute of Physics, 1973, pp. 309-313; PA1 Y. Shimada, "Domain Patterns of a Magnetic Garnet Bubble Film in an Arbitrarily Oriented Field," Journal of Applied Physics, Vol. 45, No. 7, July 1974, pp. 3145-3158; PA1 A. Hubert et al., "Effect of Cubic, Tilted Uniaxial, and Orthorhombic Anisotropies on Homogeneous Nucleation in a Garnet Bubble Film," Journal of Applied Physics, Vol. 45, No. 8, August 1974, pp. 3562-3571; and PA1 R. Wolfe et al., "Reduction of the Apparent Anisotropy of Bubble Garnet Films under Aluminum Metallization," AIP Conference Proceedings No. 29 (Magnetism and Magnetic Materials--1975), American Institute of Physics, 1976, pp. 117-118.
Methods of this latter type have been associated with the term "sea of bubbles" which is descriptive of a preferred bubble pattern as observed upon release of a magnetic field. The above-cited references show that there has been experimentation with regard to the direction in which a magnetic field is applied. Appropriate choice of such field direction is in the interest of a reliable determination of the magnetic anisotropy field, and it was found in some instances that it may be preferable to choose a field direction which deviates by a few degrees from an in-plane direction. What has been lacking, however, is a systematic approach to the optimal choice of an angle of elevation of the magnetic field to achieve reliability comparable to that of a resonance method.